To remove toxic materials from exhaust gases discharged from automobile engines, etc., a ceramic honeycomb structure 1 as shown in FIG. 1 is used for an exhaust-gas-cleaning catalyst converter comprising a ceramic honeycomb structure, and a particulate-matter-capturing filter.
The production of a ceramic honeycomb structure comprises the steps of, for example, mixing and blending ceramic powder such as a cordierite-forming material powder with a binder, a pore-forming material, water, etc. to form a moldable ceramic material; extruding the moldable ceramic material through a extrusion-molding die to form a ceramic honeycomb green body having a honeycomb structure comprising a peripheral wall 11 and flowing paths 4 partitioned by cell walls 3; removing water, etc. from the ceramic honeycomb green body; and sintering it in a furnace. In the sintering step, a molding aid such as a binder, etc. are removed from the green body, providing a ceramic honeycomb structure having predetermined shape and strength and comprising cell walls with fine pores.
When as large a ceramic honeycomb structure as having an outer diameter of 150 mm or more and a length of 150 mm or more, or a ceramic honeycomb structure comprising cell walls as thin as 0.2 mm or less is produced, a green body does not have sufficient strength, suffering such problems as the deformation of cell walls near a peripheral layer by its own weight when extruded, and failure to obtaining predetermined strength after sintering.
To solve such problems, JP 2004-148791 A discloses a method for producing a ceramic honeycomb structure by the extrusion of a moldable ceramic material, drying and sintering, comprising the steps of removing a peripheral portion of a ceramic honeycomb body by machining after drying, and coating the peripheral surface with a peripheral layer after sintering. JP 2004-148791 A describes that the removal of the peripheral portion by machining is conducted by cutting a dried ceramic honeycomb body fixed to a lathe and rotated at 260 rpm with a cemented carbide bite attached to a carrier at a cutting depth of 5 mm and a feed speed of 1.0 mm/second. JP 2004-148791 A describes that the machined ceramic honeycomb body has part of cell walls removed to have grooves open on the peripheral surface, so that cracking does not easily proceed during sintering, and that the peripheral layer is not easily peeled from the honeycomb body, resulting in a honeycomb structure having excellent isostatic strength.
WO2004/078674 A describes a method for producing a ceramic honeycomb structure by the extrusion of a moldable ceramic material, drying and sintering, comprising the steps of drying a green body, alternately plugging one or the other end portion of each cell open on both end surfaces, removing a peripheral portion (peripheral wall and part of cell walls) by machining with a lathe after sintering, and coating the exposed peripheral surface with a peripheral layer. WO2004/078674 A describes that a ceramic honeycomb structure thus obtained is free from breakage when canned, and has excellent heat shock resistance.
Though JP 2004-148791 A and WO 2004/078674 A describe a step of machining a peripheral portion of a ceramic honeycomb body by a lathe in the production of a ceramic honeycomb structure, they fail to describe detailed machining conditions. Generally, when a peripheral portion of a cylindrical body such as a ceramic honeycomb body is machined, one end portion 19 of a ceramic honeycomb body 10 is fixed to a lathe 90 with a chuck 91 as shown in FIG. 12(a), a peripheral portion 12 (hatched portion) is removed, and the end portion 19 not machined because of chucking is then cut off at a position A shown by the dotted line in FIG. 12(b), providing a ceramic honeycomb body of a predetermined length.
However, a ceramic honeycomb body used for cleaning an exhaust gas generally has a cell structure constituted by porous cell walls, with low strength because of high porosity. Accordingly, when its peripheral portion is removed by machining in a chucked state, a chucked portion of the ceramic honeycomb body may be broken by a load during machining. Particularly a high-porosity honeycomb body is more easily broken because of lower strength. Also, when the ceramic honeycomb body is sintered, its strength is reduced because a binder is removed from the green body, breakage occurs more easily. Further, when the ceramic honeycomb body is chucked for machining, a portion not machined because of chucking should be cut off, resulting in a low yield.